1. Technical Field
The present invention generally relates to cascade amplifier, and more particularly to low noise cascade amplifier.
2. Description of the Prior Art
The integrated circuit always comprises an amplifier circuit for amplifying an input signal. In general, the metal-oxide semiconductor device (MOS) or the cascode can be the amplifier circuit. Referring to FIG. 1, what is shown is a circuit diagram for the MOS as an amplifier according to the prior art. The amplifier comprises a MOS M1 and a current source I, wherein the source of the MOS M1 is grounded, the gate of the MOS M1 connects to a bias voltage VI, and the drain of the MOS M1 connects to the current source I to form a common source MOS. Although the MOS M1 can be used to amplify the input signal, the resulting gain is always too low to be a fine amplifier, and leads to the limited application of the amplifier.
Referring to FIG. 2, what is shown is a circuit diagram for a cascode device according to the prior art. The cascode device 10 comprises a common source transistor M1 and a common gate transistor M2, and the gain of the cascode device 10 is higher than the common source MOS as shown in FIG. 1.
In addition, the triple cascode device 20 comprises three transistors in series to improve the gain. As shown in FIG. 3, the triple cascode device 20 comprises a common source transistor M1, a common gate transistor M2, and a common gate transistor M3.
Referring to FIG. 4, what is shown is a gain-frequency diagram for a cascode device and a triple cascode device according to the prior art. In this embodiment, the transistor M1 is a 16-finger NMOS with a total gate width of 40 um, the transistor M2 is a 24-finger NOMS with a total gate width of 60 um, and the transistor M3 is a 32-finger NMOS with a total gate width of 80 um. Furthermore, the dashed line is the curve of the gain (MSG/MAG)-frequency of the triple cascode device 20, and the continuous line is the curve of the gain (MSG/MAG)-frequency of the cascode device 10, as shown in FIG. 4, wherein the MSG is the maximum stable gain, and the MAG is the maximum available gain.
The MSG of the triple cascode device 20 is 21.5 dB, and the MSG of the cascode device 10 is 15.3 dB, while the operating frequency is 40 GHz. In another words, the gain of the triple cascode device 20 is higher than that of the cascode device 10 or the common source MOS. Moreover, the triple cascode device 20 has the following advantages: high input impedance, high output impedance, high gain, compact size, and so forth.
Although the triple cascode device 20 provides the above advantages, it also produces noise, which limits the application of the triple cascode device 20. Referring to FIG. 5, what is shown is a noise-frequency diagram for a cascode device and a triple cascode device according to the prior art. The dashed line is the curve of the noise (Nfmin, minimum noise figure)-frequency of the triple cascode device 20, and the continuous line is the curve of the noise (Nfmin)-frequency of the cascode device 10.
The Nfmin of the triple cascode device 20 is 3.5 dB, and the Nfmin of the cascode device 10 is 2.8 dB, while the operating frequency is 40 GHz. Therefore, the triple cascode device 20 cannot be a suitable low noise amplifier (LNA) for a wireless transceiver due to its high noise figure.